1. Field of the Invention
The present invention relates to a method of manufacturing semiconductor devices, and particularly to a method of forming refractory metal films of high quality at a high speed.
2. Description of the Prior Art
High integration in a semiconductor device can be realized by micronizing its component elements. For instance, a 1M-bit DRAM or a 256K-bit SRAM is presently manufactured based on a design rule of 1 to 1.2 .mu.m. In the future, the design rule of semiconductors will be micronized further to the order of sub-micron.
Such a micronization causes many problems in processing and manufacturing the semiconductor devices. Taking wiring for instance, the width of wiring becomes smaller and the length thereof longer, due to the micronization of design rule. In addition, as the number of active elements in a semiconductor device increases, the number of locations which shall be connected electrically is increased to enlarge the aspect ratio (contact depth/contact width) of each contact hole of the semiconductor device. As a result, wiring using a material of normal Al/1%Si tends to frequently cause problems such as open defects (disconnection of wiring), discontinuity in the bottoms of connection holes, an increase in contact resistance, and electromigration, thereby deteriorating the quality and reliability of wiring.
To solve the problems, there is a demand for new wiring materials and new wiring structures. For instance, to prevent the contact resistance from increasing, a diffusion barrier layer made of refractory metal or a titanium nitride (TiN) film/titanium (Ti) structure is disposed between aluminum (Al) or Al/Si alloy and a silicon (Si) substrate to provide a wiring structure of the refractory metal film. The refractory metal films or their silicide films are thought to be potent as one of new wiring materials. Namely, films made of refractory metals such as tungsten (W), titanium (Ti), molybdenum (Mo), and their silicides are used as diffusion barrier layers of the contacts of semiconductor devices, or as gate electrodes of field-effect transistors (FETs). To form a film from a refractory metal or from its silicide, a high-speed sputtering method is usually adopted. Also, a chemical vapor deposition (CVD) method has been tried for the same purpose. When the CVD method is used to form a thin film under a reduced pressure of several torrs (a reduced-pressure CVD method) a so-called roundabout phenomenon of reactive gas forms a refractory metal film on the bottom of a groove having a large aspect ratio, and a film thickness of the refractory metal formed on the bottom is the same as that of a film formed on a flat portion of the surface of a substrate. Therefore, for the metalization of a VLSI designed according to a fine design rule and having wiring of small pitch and space as well as having narrow contact holes, the reduced-pressure CVD method is a remarkable effective method in forming thin films.
Particularly, a tungsten (W) selective CVD method which forms a tungsten (W) film on only a particular region of a substrate can simplify the processes of semiconductor device manufacturing and produce reliable tungsten films. Therefore, the tungsten selective CVD method has many practical advantages compared to other thin film forming methods.
The tungsten selective CVD method uses a mixed gas made of a halide of tungsten (usually, tungsten hexafluoride (WF.sub.6)) and hydrogen (H.sub.2 as a source gas to selectively form, according to the CVD method, a tungsten film only on refractory metal such as silicon, aluminum or its alloy, and tungsten, but not on an insulating film such as a silicon oxide film.
Usually, such a tungsten selective CVD method may realize a high-speed deposition to a certain extent, but not provide films of good quality so that, in some cases, the method may not be used in a wiring process.
Further, the tungsten selective CVD method has a limit in temperature at which a stable deposition speed is obtainable. For instance, it will be about 300.degree. C. to 340.degree. C. when the method is performed under a heating lamp in a cold wall type reactive furnace. Generally, it is difficult to strictly maintain the temperature at a constant value because of a wide error range. If the above-mentioned temperature range is not kept, it will be difficult to control the deposition speed.
According to another tungsten selective CVD method which is adopted widely and uses a WF.sub.6 -H.sub.2 system, the deposition of tungsten is always made by the reduction reaction of WF.sub.6 with substrate materials such as silicon. The deposition of tungsten causes a so-called "biting" which is a consumption of substrate materials. If the "biting" is caused excessively when tungsten is deposited on a portion of silicon substrate where a diffusion layer is formed, the diffusion layer tends to be destroyed by the "biting". As mentioned, the selective tungsten CVD method using the WF.sub.6 -H.sub.2 system hardly suppresses the consumption of substrate materials with good reproducibility. On the other hand, with the normal CVD method, it is difficult to maintain good selectivity of locations at which thick tungsten films are formed, and a practical high deposition speed.